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ISDN CrossOver Cable

Why can't I connect my PBX to my SBC or Voice Gateway?

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Here's a little gotcha for you. Sometimes, you may want to connect your PBX to your Session Border Control (SBC) or Voice Gateway, via E1/T1…and for whatever reason you just can't get the interface to come up.

Sometimes you need a ISDN CrossOver cable in these scenarios - and guess what, this is
not the same as a normal Cat5 Cross Over. They're different pin configurations.

A normal Cat5 cable is straight-forward connection, with a 1:1 Pin correlation. The colour scheme I normally use is (starting from the left, with the plug facing
away from you):

Striped-Orange
Solid Orange
Striped-Green
Solid Blue
Striped-Blue
Solid Green
Striped Brown
Solid Brown

ISDN CrossOver
The ISDN Cross-Over swaps pairs 1/2 and 4/5, so the pin layouts you're now interested in would be:

1 to 4
2 to 5
4 to 1
5 to 2

If you're making the cables then don't try and cut down the cable to those cables only - it's nearly impossible to make the cable! Just leave the other pins straight through. So using the above colour scheme the layout would be:


ISDNCrossOver

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